TRENDS IN SYSTEMATIC BOTANY
77
more and Delisle (1939); and many more may be found in the bibliographies of Anderson (1949, 1953) and Heiser (1949a).
The significance of hybridization is now generally recognized, not only
in its bearing on the composition of natural biotas, but as a potent force
in the great bursts of evolutionary activity that are known to have occurred from time to time (Anderson and Stebbins, 1954). One of the
factors that may increase the rate of natural hybridization, or may, in
fact, induce it, is the disturbance of the habitat, whether by man or some
other agency. A great increase in the frequency of hybrid derivatives
in disturbed areas speaks for the evolutionary importance of this process
in the bringing together of diverse genetic types and the opening up
of new ecological niches to receive their new kinds of progeny.
As evidence of the influence of hybridization in the evolution of the
angiosperms, Stebbins (1947) calls attention to the following three
points: (a) approximately half of the species of flowering plants are
estimated to be polyploids; (b) the great majority of these are allopolyploids (amphiploids); and (c) there is a reticulate pattern of relationship among the families and orders of flowering plants, from primitive to
complex, that is believed to be due to interbreeding between the angiosperm lines during the early stages of their divergence.
Barriers to Interbreeding.
In sexually reproducing, cross-fertilizing organisms, populations which are unlike may retain their identity if gene
exchange between them is prevented in some way. This may be done
by geographic isolation or by an isolating mechanism within the organism itself. Races of the same species are often prevented from interbreeding merely by geographic isolation; where their ranges overlap free
hybridization takes place and a zone of intermediates is formed. By
definition, races are open systems capable of exchanging genes. From this
it follows that races (often the equivalents of subspecies) of the same
species are ordinarily confined to different regions. Were they cohabitant,
they would soon lose their identities through hybridization and introgression. When an internal isolation mechanism is finally set up between
them, races are able to coexist in the same territory. At this stage in the
evolutionary process, the races are in reality at the species level in the
genetic sense, and, if their morphological differences are sufficiently pronounced to warrant it, they may be at the species level in the taxonomic
sense, too.
Races or subspecies are held apart spatially by significant geographic
barriers or presumably by natural selection operating on physiologic
differences that adapt them to somewhat different environments. Frequently they run together in parts of their distribution and thereby give
77
more and Delisle (1939); and many more may be found in the bibliographies of Anderson (1949, 1953) and Heiser (1949a).
The significance of hybridization is now generally recognized, not only
in its bearing on the composition of natural biotas, but as a potent force
in the great bursts of evolutionary activity that are known to have occurred from time to time (Anderson and Stebbins, 1954). One of the
factors that may increase the rate of natural hybridization, or may, in
fact, induce it, is the disturbance of the habitat, whether by man or some
other agency. A great increase in the frequency of hybrid derivatives
in disturbed areas speaks for the evolutionary importance of this process
in the bringing together of diverse genetic types and the opening up
of new ecological niches to receive their new kinds of progeny.
As evidence of the influence of hybridization in the evolution of the
angiosperms, Stebbins (1947) calls attention to the following three
points: (a) approximately half of the species of flowering plants are
estimated to be polyploids; (b) the great majority of these are allopolyploids (amphiploids); and (c) there is a reticulate pattern of relationship among the families and orders of flowering plants, from primitive to
complex, that is believed to be due to interbreeding between the angiosperm lines during the early stages of their divergence.
Barriers to Interbreeding.
In sexually reproducing, cross-fertilizing organisms, populations which are unlike may retain their identity if gene
exchange between them is prevented in some way. This may be done
by geographic isolation or by an isolating mechanism within the organism itself. Races of the same species are often prevented from interbreeding merely by geographic isolation; where their ranges overlap free
hybridization takes place and a zone of intermediates is formed. By
definition, races are open systems capable of exchanging genes. From this
it follows that races (often the equivalents of subspecies) of the same
species are ordinarily confined to different regions. Were they cohabitant,
they would soon lose their identities through hybridization and introgression. When an internal isolation mechanism is finally set up between
them, races are able to coexist in the same territory. At this stage in the
evolutionary process, the races are in reality at the species level in the
genetic sense, and, if their morphological differences are sufficiently pronounced to warrant it, they may be at the species level in the taxonomic
sense, too.
Races or subspecies are held apart spatially by significant geographic
barriers or presumably by natural selection operating on physiologic
differences that adapt them to somewhat different environments. Frequently they run together in parts of their distribution and thereby give